The correct answer is D. Both B and C.
The power factor of a circuit is the ratio of the real power to the apparent power. The real power is the power that is actually used by the load, while the apparent power is the total power that is drawn from the source. The power factor is a dimensionless number between 0 and 1. A power factor of 1 indicates that all of the apparent power is being used by the load, while a power factor of 0 indicates that no apparent power is being used by the load.
In a purely resistive circuit, the current and voltage are in phase, so the power factor is 1. In a purely inductive circuit, the current lags the voltage by 90 degrees, so the power factor is 0. In a purely capacitive circuit, the current leads the voltage by 90 degrees, so the power factor is also 0.
In a circuit that contains both resistance and inductance, the current and voltage are not in phase, so the power factor is less than 1. The power factor can be improved by adding capacitance to the circuit. The capacitance causes the current to lead the voltage by a smaller angle, which increases the power factor.
The power factor is important because it affects the efficiency of the power system. A low power factor causes the power company to have to generate more power than is actually being used, which increases the cost of electricity.